PETROGRAPHY AND GEOCHEMISTRY OF THE HOLOCENE INCONSTANCE LAVA FLOW, HOTLUM CONE, MT. SHASTA, CALIFORNIA

The Hotlum Cone is the youngest eruption phase of Mount Shasta, which encompasses eruptions less than 8.5ka and makes up the current summit. During its time of activity the Hotlum Cone erupted several effusive lava flows down the northern and eastern flanks of Mount Shasta. This study examines samples from the Inconstance Creek lava flow, on the eastern side of Mount Shasta. Sampling was completed strategically with three samples taken along the levees of the flow and three from the center of the flow, to assess compositional zoning of the lava. The mineralogy of the Inconstance Lava consists of phenocrysts of plagioclase, orthopyroxene (OPX), and clinopyroxene (CPX) with a fine-grained plagioclase groundmass. Gabbroic xenocrysts composed of interlocking clusters of plagioclase, OPX, and CPX are also seen both in hand-sample and thin-section of all six samples. Disequilibria textures such as oscillatory zoned plagioclase, dusty-sieved plagioclase, and reversely zoned plagioclase with andesine cores and labradorite rims are also observed. Whole-rock analysis via X-Ray Fluorescence (XRF) indicates that all six samples plot along the boundary between dacite and andesite when SiO₂ and K₂O + NaO₂ are normalized and plotted on a Total Alkali versus Silica diagram. Major element concentrations of lavas do not vary significantly between samples. However, Cr and Ni concentrations are highest in lava erupted first (91 and 78 ppm, respectively) and steadily decrease to 66 and 61 ppm, respectively in subsequently erupted lava. These results imply that the Inconstance Creek lava flow may have been erupted from a compositionally zoned magma reservoir.